Genera 4, species 37 (4 genera, 19 species in the flora , 2 or possibly 3 of these introduced): nearly worldwide, very rare in regions with high or very low precipitation; not in Greenland or Aleutian Islands.

In this treatment the terms upper / lower and above / below are always used in relation to the position of the frond in the water. The positions toward the base and the apex of the frond are called proximal and distal, respectively.

The "leaf" in the Lemnaceae does not correspond to the leaf of higher plants. It is supposed to consist of a stem in the proximal part (from base to node) and a leaf in the distal part (from node to apex). In Lemnaceae literature it is called a frond. Some authors hold that the flower of the Lemnaceae corresponds to an inflorescence consisting of 1 to 2 male flowers (anthers) and 1 female flower (ovary).

With respect to chromosome number, hundreds of Lemnaceae clones were counted, and three levels of cytological variations were identified: (1) intra-individual variation (aneusomaty and / or mixoploidy, (2) intra-populational variation (aneuploidy or polyploidy), and (3) "racial" differentiation (K. Urbanska-Worytkiewicz 1980). Therefore, many chromosome numbers within a species were counted even if only North American plants are listed.

Lemnaceae are the smallest and most reduced flowering plants. Therefore, closely related species have but few distinguishing characteristics that are easy to recognize. In addition, most of these features are strongly modificable and overlap considerably. Occasionally Lemnaceae have small, 1--5-celled papillae on the upper surface of the frond. Some species have turions (compact fronds reduced in size and structure, filled with starch grains, forming under unfavorable conditions). Easily recognized by their darker color, tTurions remain for several days with the mother frond before they sink to the bottom.

Because of the rarity of flowering and fruiting, only vegetative characteristics are available in most cases. Lemna gibba, L. aequinoctialis, and L. perpusilla are the only American species fruiting rather frequently. The smallness of the plants requires good binoculars magnification and some technical preparations for identification. To analyze anatomic structures (e.g., number of .veins or extension of air spaces), transparent slides are necessary. Fronds preserved in 70% ethanol become transparent. Dried fronds must be boiled first in 70% ethanol and treated afterwards with 10% NaOCl to clear them. Preserving them in ethanol, however, removes pigments that are sometimes important for determination. Under optimal growth conditions the typical anthocyanin pattern of some species does not develop. Therefore, one should observe the species in another season or cultivate it under different conditions. Another difficulty of determination occurs because several (up to 10) Lemnaceae species very often grow together in nature. Therefore, many collection samples contain more than one species, some of which may not be recognized as different at first glance.

Lemnaceae are easily distributed by birds over short distances. In many places, they live only so long as conditions are favorable. Afterwards they disappear. A species of a southern area might suddenly occur farther north and remain there for one or .several years. If .the species is exposed to water fed by a warm spring, it might persist far beyond expectations. The distribution maps of the Lemnaceae show the area where the species once was collected and do not represent the actual distribution area, which might be considerably smaller and change within a few years. Generally, most Lemnaceae species have expanded during the last years because of the warming of the climate and eutrophication of the waters.

Lemnaceae have a high productivity (some species can double in number within 24 hours) and a very high percentage of amino acids (up to 45% of dry weight). They are used in many regions as food for poultry, pigs, and cows. Wolffia fronds are eaten as a vegetable in southeastern Asia; Lemna gibba is cultivated in Israel for use as a vegetable and salad. Lemnaceae are also used for waste-water purification and as test and indicator plants.

Fronds flat (linear, ribbon-, sabre-, or tongue-shaped, or ovate), with air spaces; daughter fronds from terminal flat pouch at mother-frond base; flower(s) in cavity at side of median line of upper frond surface

Fronds 3-dimensional (globular, ovoid, or boat-shaped), without air spaces; daughter fronds from terminal conic pouch or cavity at mother-frond base; flower in cavity on median line of upper frond surface